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Re: SISG and primary voltage.

Original poster: Vardan <vardan01@xxxxxxxxxxxxxxxxxxxxxxx>

Hi Aaron

At 09:32 AM 5/24/2006, you wrote:
I've been following the SISG threads closely...even
ordered parts to play with!  But my experience is
primarily with classical SGTCs, and my grasp of the
theory largely limited to rules of thumb, so I'm a
fish mostly-out-of-water with all this solid-state
stuff! :-((

It's pretty easy and you really can "just wire it up" without knowing how it works. there are boards, kits, and probably fully assembled things in the future...

In particular, it occurs to me that I now no longer
understand why one would opt for a HV transformer with
SISGs around.  I've got more ~2kV MOTs than I know
what to do with, but why even go *that* high?  Why not
take an OLTC-ish approach and charge the caps straight
from the rectified mains?

I originally was trying to make an OLTC little coil using SIDACs. But they just ended up have too little current drive and power dissipation. So I tried to get and IGBT to take over the load once they fired... It worked out good!!! :o)))

At first I cold only get about 350 amps with the old IGBTs, so I went to higher voltage to get more power with lower current. But the new 800+ amp IGBTs solved that. Of course, those giant 5000+ amp bricks on E-bay would "really" solve that :-)) But at the time I had the 800 amp IGBTs and "being able" to put sections in series for higher voltage was in important thing that needed to be tested.... So, for one reason or another, I ended up using MOTs and all....

However, there is no reason you could not use a single large IGBT and simply adjust the number or value of the SIDACs to fire at any voltage you wish. I was trying for 120BPS since that seems to work best so the firing voltage would be about 160 volts. But I got all off track from that when the SISG thing took over. I hope to get back to it now ;-))

The IGBT drive circuit for the SISG should be able to drive any IGBT "as is". I have not looked at the really large bricks, but maybe a few resistor changed at most... Probably no change is need though.

With SGTCs, the importance
of quenching and the difficulty of quenching at low
voltages seems to dictate that some amount of HV-ness
is *required*, not just *good*.  With the quenching
argument gone, can somebody attempt to explain (for
the non-EEs among us--me included!) why one might opt
for a HV SISG coil versus a LV one?  I'm sure there
*are* reasons for one over the other (i*i*r losses,

Pretty much "only" I^R losses and perhaps the difficulty of charging big low voltage primary caps off the line. But no big problem there other than it has not been done "yet"... The SISG has only been really around for about a "week" now ;-)))

, but the factors aren't all obvious to me.
Some people are all about efficiency, but if similar
output can be achieved in the SISG coil by losing the
HV transformer and bearing some IGBT heating instead,
that would also be "cool" :-)

I too have no love for the big iron things... But at the time it worked out as it did and I "went with the army I had". But many many other variations are possible!! Mine is just "one" example...

I wonder if there are any cheap switching power supplies out there that would work too... I hear the new microwave ovens and NSTs are switching now...


Thanks, Terry and company, for proving that modern
technology might actually be able to *simplify* coil
construction :-))  Always thought it had to be so, but
I've seen relatively few examples of it until now.